This equation computes the probability that either one of two events will both happen.  The inputs to this equation are the probability of event A occurring and the probability of event B occurring. 

Notes

The probability that one of two events, event A or event B, may occur is the sum of their probabilities.

If there are half boys and half girls in a group of  ten students, the probability of picking a girl, P(A), is 0.5, or 5 in ten.

The probability of picking a boy, p(B), is 0.5, or 5 in ten.  Therefore the probability of picking either a boy or a girl is p(A or B) = 0.5 +0.5 = 1.